Formatting can hugely change the way text is read. It's quite common for the spacing between sections to be derived from the formatting. So removing the formatting can result in section headings with zero space from the preceeding paragraph but a manual newline between it and the next paragraph. Thus the headings essentially disappear into the preceeding text.

I've had countless other examples over the years where "paste without formatting" involved lots of manual editing of white space. Bullet and numbered lists sometimes vanish which rather matters if the text says "see point 3 above"